Abstract

A new, exceptionally well-preserved articulated skull roof and braincase of Melvius chauliodous, from the Kirtland Formation of New Mexico, reveals new features that provide detailed osteological information for Melvius, thus allowing for a reassessment of characters that defines the genus, its two recognized species M. chauliodous and M. thomasi, and its phylogenetic relationships within Vidalamiinae. The new features include the presence of dermopterotic rays (‘ribs’) and parietal ray bundles, previously unknown for both species of Melvius. The presence of elongated dermopterotic rays, a feature previously considered to be distinctive for the genus Calamopleurus, and tribe Calamopleurini, is now documented in Melvius, but these rays differ in their point of origination at the posteromedial corners of both dermopterotics. Bundles of short fan-shaped rays emanating from below the posterolateral corners of the parietals are also documented in both species of Melvius. Melvius was previously placed in the tribe Vidalamiini (Subfamily Vidalamiinae), but because it shares some unique features with members of the tribe Calamopleurini and the genus Calamopleurus, its placement within Vidalamiini, and the recognition of these two distinct clades (tribes) within Vidalamiinae, are not supported in our phylogenetic analysis. Previous characters distinguishing Melvius species, particularly differences in postinfraorbital ornamentation between M. chauliodous and M. thomasi, are considered dubious. However, other features may serve to determine interspecific variation or differences. For the present, we retain both species until more definitive material of M. thomasi is described.